In networks such as Internet Protocol (IP) networks, when a host seeks to transmit a sizable amount of date to a target destination in the network the data is transmitted as a set of datagrams. In most cases it is more efficient to send the data using the largest possible datagrams such that these datagrams do not have to be fragmented at any point along the path from the host to the destination. Fragmentation occurs on hops along the route where an MTU is smaller than the size of the datagram. To avoid such fragmentation the MTU for each hop along a path from a host to a destination must be known to select an appropriate datagram size. Such MTU determination along a specific path is referred to as path MTU discovery or PMTU discovery (PMTUD).
Current PMTUD mechanisms are primarily targeted to work on point-to-point paths, i.e. unicast paths. These mechanisms use packet fragmentation control by disabling fragmentation of the probe packet. As result, a transient node that cannot forward a probe packet due to it being too large for the MTU of the next hop, instead sends an error message back to the ingress node of the path being tested. If the probe packet successfully traverses the path then the egress node responds with a positive notification. Thus, through a series of iterations where the probe packet size is incrementally reduced and/or increased, the ingress node of the path can discover a PMTU of the particular path. Packet fragment control works in Internet Protocol v4 Networks, however, IPv6 networks do not support fragmentation.
Bit indexed explicit replication (BIER) is a multicast forwarding architecture that is designed to efficiently handle multicast without requiring an explicit tree-building protocol or the maintenance of flow state by intermediated nodes. Routers in network that support BIER are referred to as Bit-Forwarding Router (BFR). A BIER domain consists of BFRs that are uniquely identified by BFR-id. An ingress border router, a Bit Forwarding Ingress Router (BFIR), inserts a Bit Mask Set (BMS) into a packet. Each targeted egress node, a Bit Forwarding Egress Router (BFER), is represented by Bit Mask Position (BMP) in the BMS. A transit or intermediate BIER node, a BFR, forwards BIER encapsulated packets to BFERs, identified by respective BMPs, according to a Bit Index Forwarding Table (BIFT) built based on domain-wide Bit Forwarding Tree (BFT).
The existing solutions for PMTUD are inefficient for point-to-multipoint paths constructed for multicast traffic. Probe packets must be flooded through the whole set of multicast distribution paths over and over until the very last egress responds with a positive acknowledgement.